Method for measuring component of a gaseous emission

ABSTRACT

A modified universal exhaust gas oxygen sensor, referred to herein as a CEGA sensor, is provided which can be used to measure the concentration of a variety of components of a gaseous fuel emission including CO, CO 2 , O 2 , H 2 , and H 2 O. The CEGA sensor-employs at least one additional electrode on a ceramic substrate which possess a different catalytic activity relative to the electrodes that normally found on a UEGO sensor. The ceramic substrate may be made of any suitable ceramic and is preferably made of zirconia. The difference in catalytic activity between the additional electrode(s) and the electrodes native to the UEGO sensor create an oxygen gradient which enables a measure of combustion completeness to be calculated. In combination with an air/fuel ratio measured by the sensor, the concentrations of different components in the emission can be calculated. Several methods, devices and systems which can be used with various types of ceramic sensors including a CEGA sensor in order to improve their performance are also provided.

This application is a continuation of application Ser. No. 08/902,552,filed Jul. 29, 1997, now U.S. Pat. No. 6,254,950, which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to sensors for use in detecting gaseouscomponents and more particularly to ceramic sensors for use in analyzingcombustion emission components.

BACKGROUND OF THE INVENTION

A variety of sensors have been developed for detecting different gaseouscombustion emission components. Examples of the different gaseouscomponents which these sensors can detect include, but are not limitedto oxygen (O₂), carbon monoxide (CO), carbon dioxide (CO₂), hydrocarbons(HC), and nitrogen oxides (NO_(x)). These sensors can be used in avariety of devices including, for example, automotive engines, dieselengines, gas turbine engines, jet engines, power plants, furnaces, andbarbecues. Many of these gaseous components are hazardous.

Information derived from these sensors can be used for a variety ofpurposes. Data from the sensors can be used for feedback control ofdifferent aspects of a device which is producing a gaseous emission.Alternatively, these sensors can simply be used to monitor the contentof the emission. For example, these sensors can be used as a componentof an on-board, OEM emissions control system for an automotive engine oras an off-board emissions measuring device used for inspection andmaintenance, for example as a tool for an automotive mechanic.

A need exists for sensors which can detect a wide array of gaseouscomponents. For example, a need exists for the sensor which candetermine the concentrations of oxygen, carbon monoxide, carbon dioxide,hydrocarbons, and nitrogen oxides in a sample. The sensors should have ahigh signal-to-noise ratio and thus be able to accurately determine theconcentrations of various components of a gaseous sample. The sensorsshould be simple, reliable, and inexpensive to manufacture. These andother objectives are provided by the sensors, devices, and methods ofthe present invention.

SUMMARY OF THE INVENTION

The present invention relates to a modified universal exhaust gas oxygensensor, referred to herein as a CEGA sensor, which can be used tomeasure the concentration of a variety of components of a gaseousemission including CO, CO₂, O₂, H₂, and H₂O. The CEGA sensor employs atleast one additional electrode on a ceramic substrate which possess adifferent catalytic activity relative to the electrodes that arenormally found on a UEGO sensor. The ceramic substrate may be made ofany suitable ceramic and is preferably made of zirconia.

The difference in catalytic activity between the additional electrode(s)and the electrodes native to the UEGO sensor create an oxygen gradientwhich enables a measure of combustion completeness to be calculated.Combustion completeness is a parameter quantifying the degree to whichthe gaseous emissions of combustion are in chemical equilibrium. Incombination with an air/fuel ratio measured by the sensor, theconcentrations of different components in the emission can becalculated.

A method is also provided for measuring concentrations of components ofa gaseous emission by measuring an air/fuel ratio using a ceramicsensor, measuring combustion completeness using the ceramic sensor anddetermining concentrations of components of a gaseous emission based onthe measured air/fuel ratio and measured combustion completeness. TheCEGA sensor of the present invention enable these functions of themethod to be performed by a single sensor.

In one regard, the CEGA sensor is an improved universal exhaust gasoxygen sensor (UEGO) for measuring properties of a gaseous emissionwhich includes at least one oxygen pumping cell and a sensing cell incontact with a detection cavity, the sensing cell including a ceramic ingas communication inside the detection cavity, a first electrode incontact with ceramic positioned inside the detection cavity, and asecond electrode in contact with the other side of the ceramic, a firstvoltage potential externally applied between the first and secondelectrodes for pumping oxygen across the ceramic into and out of thedetection cavity, the first voltage potential controlled by a secondvoltage potential formed across a third and fourth electrode of thesensing cell, an air/fuel ratio measurement of the gaseous emissionbeing obtainable from the current passing between the first and secondelectrodes, the improvement comprising the addition of a fifth electrodewhich has a different catalytic activity than the first electrodepositioned inside the detection cavity in contact with the pumping cellceramic, a third voltage potential externally applied between the firstelectrode and either the second electrode or a sixth electrode locatedon the same side of the pumping cell ceramic as the second electrode,the third voltage potential controlled by a fourth voltage potentialformed between the first and fifth electrodes, a measure of combustioncompleteness being obtainable from the current passing between the firstand the sixth electrodes.

In one particular embodiment of a CEGA sensor, the sensor includes

a detection cavity;

a diffusion passage across which the gaseous emission enters thedetection cavity; an oxygen pumping cell defining a portion of thedetection cavity formed of a ceramic substrate and a first electrode inthe detection cavity and a second electrode outside the detection cavityfor pumping oxygen into and out of the detection cavity across theceramic substrate to maintain a target oxygen level concentration in thedetection cavity, an air/fuel ratio measurement of the gaseous emissionbeing obtainable from current passing between the first and secondelectrodes; and

a sensing cell defining a portion of the detection cavity formed of aceramic substrate, the sensing cell including

a third electrode within the detection cavity,

a fourth electrode outside the detection cavity, a second voltagepotential being formed between the third and fourth electrodes due to adifference in oxygen concentration across the third and fourthelectrodes, and

a fifth electrode in contact with the ceramic within the detectioncavity which has a different catalytic activity than the firstelectrode, a fourth voltage potential being formed between the fifthelectrode and the first electrode due to a difference in oxygenconcentration across the fifth electrode and the first electrode, ameasure of combustion completeness being obtainable from a currentpassing between the first and the sixth electrodes.

The present invention also relates to several methods, devices andsystems which can be used with various types of ceramic sensorsincluding the CEGA sensor of the present invention in order to improvetheir performance.

In one regard, the invention relates to a method for calibrating aceramic sensor which, as one of its functions, determines an air/fuelratio. This method can be used in combination with any sensor whichcalculates an air/fuel ratio including, but not limited to UEGO, NO_(x)and CEGA sensors.

According to the method, a ceramic sensor is operated at a constant,known air/fuel ratio. While being operated at a constant, known air/fuelratio, the pumping current (I_(pm)) of the sensor is measured. A basicrelationship which correlates the air/fuel ratio to the pumping currentfor the family of sensors to which the specific ceramic sensor belongsis then used to calibrate the sensor by comparing the measured pumpingcurrent (I_(pm)) to the expected pumping current from the basicrelationship for that air/fuel ratio (I_(p)). A transformation betweenthe measured pumping current (I_(pm)) and the current that the basicrelationship gives for a known air/fuel ratio is created. Duringsubsequent sensor usage, this transformation is used to modify themeasured pumping current to create a value which is used with the basicrelationship to obtain an air/fuel ratio that is accurate for thespecific sensor.

In one particular embodiment, the method for calibrating a ceramicsensor which, as one of its functions, determines an air/fuel ratioincludes the steps of:

operating the ceramic sensor at a constant, known air/fuel ratio;

measuring a pumping current of the sensor;

comparing the measured pumping current to an expected pumping currentfor the constant, known air/fuel ratio; and

calibrating the sensor using a basic relationship which provides theexpected pumping current for the air/fuel ratio at which the ceramicsensor was operated.

The present invention also relates to a software algorithm which can beincorporated into a system in which the sensor is used which comparesI_(pm) versus I_(p) for one or more air/fuel ratios and produces alook-up table for I_(pm) versus air/fuel ratio which can be used duringthe operation of the sensor.

The present invention also relates to a semiconductor memory devicewhich can be used in combination with or incorporated into a ceramicsensor, the memory device including logic and data for performing avariety of functions. For example, the memory device can include logicfor calibrating the sensor as well as memory for calibration data forthe sensor. The memory device can also include logic and memory forstoring usage information regarding the sensor. The memory device canalso include logic which monitors and controls the operation of thesensor. The memory device can also include logic for detecting when thesensor is being used or has been used beyond its recommended limits,e.g., temperature, time, voltage, etc. The memory device can alsoinclude a mechanism for warning the user of the improper use or overuse.

A method is also provided for correcting for temperature transients bymeasuring the temperature of the sensor; and correcting an output of thesensor based on the measured temperature. The system for operating thesensor can also include logic for adjusting the sensor's output based ona determination of the sensor's temperature.

The present invention also relates to a method for reducing noise fromleakage current from the sensor's heater by taking measurements when theheater is off or after the effects of the leakage current have reachedsteady-state, most preferably just prior to turning the heater on.

The present invention also relates to a method for reducing noise due tocoupling between the heater wires and sensing element's wires by takingsensor measurements before transitions in the heater's voltage occur.

The present invention also relates to a method for reducing noise due tothe use of a sensor impedance measuring method for determining asensor's temperature by taking measurements just before the impedancemeasuring event.

The present invention also relates to logic for performing any of theabove methods for avoiding noise by controlling when sensor measurementsare taken. The present invention also relates to logic for determiningwhether the heater duty cycle is low or high and for selecting themeasurement times based on the duty cycle.

The present invention also relates to a method for reducing noise due toa regulated voltage-type heater in a ceramic emission sensor system bymeasuring the noise due to the regulated voltage-type heater at andsubtracting the noise from the sensor signals in order to compensate forthis source of noise.

The present invention also relates to a method for improving theaccuracy of measuring oxygen-containing species in a gaseous emission inmultiple cavity sensors. According to one embodiment, the method isperformed by applying a gaseous emission to the sensor; measuring apumping current in a first cavity of the sensor which has a functionalrelationship to an air/fuel ratio of the gaseous emission; measuring apumping current in a second cavity of the sensor which has a functionalrelationship to an amount of oxygen-containing species in the gaseousemission and the air/fuel ratio of the gaseous emission; and using acombination of the measured pumping currents of the first and secondcavities to measure an amount of oxygen-containing species in thegaseous emission. This method can be incorporated into the sensor byincorporating logic and data for performing the method into the sensor.

The present invention also relates to a method for field calibratingsensors using gaseous emissions. According to one embodiment, the methodof field calibration is performed by applying a gaseous emission havinga known amount of oxygen-containing species to the sensor; measuring apumping current in a first cavity of the sensor which has a functionalrelationship to an air/fuel ratio of a model gas, measuring a pumpingcurrent in a second cavity of the sensor which has a functionalrelationship to the amount of oxygen-containing species in the gaseousemission and the air/fuel ratio of the gaseous emission; and using acombination of the measured pumping currents of the first and secondcavities and the known amount of oxygen-containing species in thegaseous emission to calibrate the sensor. This method can beincorporated into the sensor by incorporating logic and data forperforming the method into the sensor.

The present invention also relates to a method for minimizing the effectof rapid emission composition transients on the accuracy of multi-cavityexhaust sensors. According to the method, the effect of rapid emissioncomposition transients on the accuracy of a multi-cavity exhaust sensoris minimized by measuring the sensor values; detecting for an occurrenceof a rapid emission composition transient; discontinuing usage of themeasured sensor values when the rapid emission composition transient isdetected; detecting for a subsidence in the rapid emission compositiontransient; and resuming usage of the measured sensor values whensubsidence of the rapid emission composition transient is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of an UEGO sensor.

FIG. 2 provides a graph illustrating that the concentrations of CO, CO₂,O₂ and H₂ in an engine's exhaust are a function of the normalizedair/fuel ratio.

FIG. 3 illustrates an embodiment of a NO_(x) sensor.

FIG. 4 illustrates an embodiment of a CEGA sensor with two additionalelectrodes relative to a UEGO sensor.

FIG. 5 illustrates an embodiment of a CEGA sensor with one additionalelectrode relative to a UEGO sensor.

FIG. 6 illustrates the basic relationship between the pumping current(I_(p)) in a ceramic sensor and the air/fuel ratio.

FIG. 7 illustrates an embodiment of a memory device coupled to a sensorin a system according to the present invention.

FIG. 8 illustrates an alternate embodiment where the memory device isbuilt into the sensor.

FIG. 9 is a graph plotting current from a ceramic sensor (I_(p)) as afunction of temperature.

FIGS. 10A-10I illustrate a series of measuring timing patterns forenhancing the signal-to-noise ratio of the sensor.

FIG. 10A illustrates a noiseless signal (I_(p2)) in the sense thatextraneous noise has been eliminated.

FIG. 10B illustrates the duty cycling of voltage to the heater.

FIG. 10C illustrates how the signal illustrated in FIG. 10A is modifiedas a result of the leakage current.

FIG. 10C illustrates how the signal illustrated in FIG. 10A is modifiedas a result noise generated due to coupling between the heater wires andsensing element's wires.

FIG. 10D illustrates the noise effects associated with the signalillustrated in FIG. 10B.

FIG. 10E illustrates the noise effect associated with the timing of asensor impedance measurement.

FIG. 10F illustrates a low heater duty cycle.

FIG. 10G illustrates a signal with noise generated from a heateroperating with a low heater duty cycle.

FIG. 10H illustrates a high heater duty cycle.

FIG. 10I illustrates a signal with noise generated from aheater-operating with a high heater duty cycle.

FIG. 11 illustrates a NO_(x) sensor with a regulated voltage heatercontroller in a first configuration.

FIG. 12 illustrates a NO_(x) sensor with a regulated voltage heatercontroller in a second configuration.

FIG. 13 illustrates signals from a dual-cavity ceramic exhaust sensorduring engine load transients.

FIG. 14 is a plot of Vs.

FIG. 15 illustrates a software flowchart for the technique.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a modified universal exhaust gas oxygen(UEGO) sensor which can be used to measure the concentration of avariety of components of a gaseous fuel emission including CO, CO₂, O₂,H₂, and H₂O. The modified UEGO sensor, referred to herein as a CEGAsensor, has an analogous structure to a UEGO sensor but employs at leastone additional electrode on the ceramic substrate which possess adifferent catalytic activity relative to the electrodes that normallyfound on a UEGO sensor. The ceramic substrate may be made of anysuitable ceramic and is preferably made of zirconia.

The difference in the catalytic activity between the one or moreadditional electrodes in the CEGA sensor and the electrodes native to aUEGO sensor causes an O₂ gradient to be formed when the emission is notin chemical equilibrium due to an excess of either O₂ consuming or O₂generating reactions occurring in the vicinity of the electrode with ahigher catalytic activity. By monitoring the size of the O₂ gradient, ameasure of combustion completeness can be calculated. The CEGA sensor,like a UEGO sensor, is able to measure an air/fuel ratio. By comparingthe combustion completeness and air/fuel ratio measurements, theconcentrations of different components in the emission can becalculated.

The present invention also relates to several devices, methods andsystems which can be used with various types of ceramic sensorsincluding the CEGA sensor of the present invention in order to improvetheir performance.

1. EGO, UEGO & NO_(x) Type Ceramic Sensors

A variety of different ceramic gas sensors have been developed fordetecting different products in combustion emission components. Theseceramic sensors can include a variety of ceramic substrates including,for example, zirconia (ZrO₂) and titania. One type of ceramic gas sensorthat has been developed is the exhaust gas oxygen sensor (EGO). Thesesensors are used to maximize the efficiency of a catalytic converterwhich receives the emissions. For maximum converter efficiency, it mustbe fed with emissions from combustion processes operating at astoichiometric balance between air and fuel. EGO senses can only detectwhether an engine is running rich or lean of the stoichiometric point.The voltage potential (V_(s)) generated by an EGO sensor can beexpressed according to the following equation:

V _(s)=(RT/4F) In (Po ₂1/Po ₂ 2)  (I)

where:

R is the gas constant;

T is the temperature;

is the Faraday constant;

Po₂ 1 is the partial pressure of oxygen on side 1 of the sensing cellexposed to the combustion emissions; and

Po₂ 2 is the partial pressure of oxygen on side 2 of the sensing cellexposed to a reservoir of oxygen molecules of a known concentration.

Equation I has a very sharp transition value at the stoichiometric pointand hence can be used to identify this point. The sensor can also beused to measure oxygen concentrations of the emission in the vicinity ofthe stoichiometric point. While EGO sensors can detect whether an engineis operating at, above or below the stoichiometric point, the sensorcannot detect the actual air/fuel ratio of the emission.

A second type of ceramic gas sensor that has been developed is theuniversal exhaust gas oxygen sensor (UEGO). Unlike the EGO, UEGO sensorsare “linear” oxygen sensors in the sense that they can detect the actualair/fuel ratio of the emission.

FIG. 1 illustrates an embodiment of an UEGO sensor. As illustrated, thesensor includes three ceramic cells 14, 18, 33 and a porous diffusionpassage 20 into which emissions 23 enter the sensor. Two of the ceramiccells 14, 33 are used for pumping oxygen while the third cell 18 is usedas a sensing cell.

All three ceramic cells have platinum electrodes 22 attached to eachside of the ceramic substrate. When a voltage potential is placed acrossthe pumping cell's electrodes and the cell's temperature is above 300°C., oxygen is pumped through the ceramic from one side 26 (or 27) of acell to another side 27 (or 26) of the cell via the chemical reaction:

O ₂+4e ⁻←→2O ²⁻  (II)

The oxygen ions 31 generated during pumping pass through the ceramicwhile the electrons travel through external circuitry 29 connecting theelectrodes. The amount of oxygen that is pumped through the cell isdetected by the external circuitry by measuring the amount of currentgenerated, four electrons equaling a pumped oxygen molecule.

Sensing cell 18 is an EGO sensor which generates a voltage potential(V_(s)) across electrodes 33, 35 when a difference in oxygenconcentration is present across its surface. The voltage potential(V_(s)) is used to drive external circuitry 29 whose purpose is to pumpoxygen into and out of the detection cavity 28 so as to maintain astoichiometric exhaust composition therein.

The diffusion passage 20 allows combustion constituents to flow into andout of a detection cavity 28 located between one of the pumping cells 14and the sensing cell 18. Over time, the composition of components oneither side of the diffusion passage would be equal if not for thepumping of oxygen into and out of the detection cavity 28 by the pumpingcell 14 and the combustion reactions that occur inside the detectioncavity 28. By measuring the amount of oxygen pumped via the externalcurrent flow (I_(p)) into or out of the detection cavity 28 to maintaina stoichiometric exhaust composition, the exhaust's air/fuel ratio isdetermined.

When the exhaust mixture contains excess oxygen relative to astoichiometric air/fuel ratio, commonly referred to as lean ofstoichiometric, oxygen from the exhaust diffuses through the passageinto the detection cavity 28. In this case, the pumping cell 14 removesoxygen from the detection cavity 28 to create a stoichiometric exhaustcomposition.

When the exhaust mixture contains less oxygen relative to astoichiometric air/fuel ratio, commonly referred to as rich ofstoichiometric, CO and H₂ from the exhaust diffuse through the passageinto the detection cavity 28. In this instance, the pumping cell 14pumps oxygen into the detection cavity 28 which reacts with CO and H₂ tocreate a stoichiometric exhaust composition.

The sensing cell 18 has one side 30 exposed to the pumping cell 14 andanother side 32 exposed to a reference cell 33 which has a constantoxygen concentration. In some embodiments, the reference cell is passiveand is the environment outside the sensor. Alternatively, as illustratedin FIG. 1, the reference 33 cell can be an active type reference cellwhose O₂ concentration is held constant via a small pumping current.

Also shown in FIG. 1 are pumping cell control electronics 35. Thepositive side 34 of an op-amp 36 is held at approximately 0.45 V (theV_(s) target) and depending on whether the sensing cells voltageis >0.45 V (i.e., a rich mixture in the detecting cavity) or <0.45 V(i.e., a lean mixture in the detecting cavity) the op-amp 36 eithersupplies electrons (for rich mixtures) or removes electrons (for leanmixtures) from the reference cell 33. This results in O₂ either beingsupplied to the detection cavity for rich mixtures or removed from thedetection cavity for lean mixtures.

The amount of oxygen pumped by the pumping cell 14 to balance thediffusion of O₂, CO, and H₂ into the detection cavity 28 is a functionof the following parameters:

a) the diffusivity of O₂ through the diffusion passage;

b) the diffusivity of CO through the diffusion passage;

c) the diffusivity of H₂ through the diffusion passage; and

d) the concentration of O₂, CO, and H₂ in the exhaust which is afunction of the fuel's chemical composition (H:C and O:C ratios) and theair/fuel ratio, as illustrated in FIG. 2.

The diffusion coefficients are principally set by the structure of thediffusion passage. These coefficients can be determined by calibrationinstruments and are provided with the sensor. The fuel's composition(H:C and O:C ratios) can be input by the user. With this backgroundinformation, the air/fuel ratio can be calculated.

A third type of ceramic sensor that has been developed is a NO_(x)sensor. A NO_(x) sensor is essentially an UEGO sensor with an additionaldiffusion passage and cavity. See U.S. Pat. No. 5,145,566. In additionto detecting the air/fuel ratio as does the UEGO sensor, the additionaldiffusion passage and cavity of the NO_(x) sensor enables the detectionof other emission components such as CO, CO₂, H₂O and NO_(x).

An embodiment of a NO_(x) sensor is illustrated in FIG. 3. Asillustrated in FIG. 3, the sensor 40 integrates a UEGO sensor with asecond cavity 48.

The UEGO portion of the sensor 40, defined by first diffusion layer 20and detection cavity 28, acts to keep the oxygen concentration of thedetection cavity 28 at a low and constant amount by pumping oxygen intoand out of the detection cavity 28. Combustion gases enter the detectionchamber 28 via the first diffusion layer 20 as in a UEGO sensor. Bymonitoring the pumping current of the detection cavity (I_(p1)), oxygenconcentrations and air/fuel ratios are measured.

Gases within the detection cavity 28 pass through a second diffusionlayer 51 where they enter the dissociation cavity 48. In thedissociation cavity 48, a constant pumping potential is maintained whichselectively strips oxygen from NO_(x) molecules producing a dissociationcavity current I_(p2). The dissociation cavity current is proportionalto the NO_(x) concentration in the exhaust.

It is noted that although the NO_(x) sensor is described with regard tothe embodiment illustrated in FIG. 3, several other configurations ofNO_(x) known and/or possible and are intended to fall within the scopeof this invention.

2. CEGA Sensors

A modified universal exhaust gas oxygen (UEGO) sensor which can be usedto measure the concentration of a variety of components of a gaseousfuel emission including CO, CO₂, O₂, H₂, and H₂O has also beendeveloped. The modified UEGO sensor, referred to herein as a CEGAsensor, has an analogous structure to a UEGO sensor but employs one ormore additional electrodes on the ceramic substrate which possess adifferent catalytic activity relative to the electrodes that arenormally found on the ceramic substrate of a UEGO sensor.

The difference in the catalytic activity between the one or moreadditional electrodes in the CEGA sensor and the electrodes native to aUEGO sensor causes an O₂ gradient to be formed when the emission is notin chemical equilibrium due to an excess of either O₂ consuming or O₂generating reactions occurring in the vicinity of the electrode with ahigher catalytic activity. By monitoring the size of the O₂ gradient, ameasure of combustion completeness can be calculated. The CEGA sensor,like a UEGO sensor, is able to measure an air/fuel ratio. By using thecombustion completeness and air/fuel ratio measurements, theconcentrations of different components in the emission can becalculated.

CEGA sensors provide advantages over existing ceramic sensors. Forexample, CEGA sensors are less complex to build and have a greater speedof response than the sensors described in U.S. Pat. No. 5,145,566 formeasuring exhaust components including CO, CO₂, and H₂O.

Two embodiments of a CEGA sensor are illustrated in FIGS. 4 and 5. Asillustrated, the CEGA sensor is essentially a wide-range UEGO sensorsuch as the sensor illustrated in FIG. 1 where one or more electrodeshave been added. For example, the CEGA sensor illustrated in FIG. 4 hastwo additional electrodes 80 and 82. The CEGA sensor illustrated in FIG.5 has one additional electrode 80.

With regard to FIGS. 4 and 5, the electrode 80 is positioned in thedetection cavity 28 of the sensor on the same substrate as the pumpingcell electrode 26. Electrode 80 has a different (more or less) catalyticactivity than the pumping cell electrode 26. With regard to FIG. 4, theCEGA sensor also includes an electrode 82 positioned outside of thedetection cavity 28.

With regard to the CEGA sensor illustrated in FIG. 5, this CEGA sensordiffers from the CEGA sensor illustrated in FIG. 4 in that it does notinclude electrode 82. Instead, another electrode which is used in thebasic UEGO sensor, for example electrode 22, can perform the functionotherwise provided by electrode 82.

The CEGA sensor pumps oxygen into and out of the detection cavity 28 tomaintain a constant sensing cell voltage (V_(s)) 81 independent of thestoichiometry of the combustion device's exhaust 23 which enters thedetection cavity 28 through diffusion aperture 20.

Electrodes 80 and 26 have different levels of catalytic activity. If thegas entering the detection cavity 28 is not in chemical equilibrium,these different levels of catalytic activity create a difference in theoxygen concentration between electrodes 80 and 26 which can be used todetermine the degree of combustion completeness of the combustiondevice's exhaust.

The voltage potential between electrodes 26 and 80 created by thedifference in oxygen concentrations can be used to drive externalelectronics whose purpose is to pump oxygen to or away from electrode 26in order to reduce the voltage potential between electrodes 26 and 80.The polarity and amount of oxygen pumped is a function of the degree ofcombustion completeness of the exhaust gas entering the sensor and canbe measured as a function of an oxygen pumping current (I_(pe)).

The chemical composition of the emission can be determined from thedetected air/fuel ratio and degree of combustion completeness asdescribed below. First the total pumping current (I_(p)+I_(pe)) ismeasured and used to determine the air/fuel ratio. Determination of theair/fuel ratio can be performed according as is known in the art withregard to UEGO sensors.

At any given air/fuel ratio, the percentage of component X can berepresented by the equation:

Xi=Xi,afr+Gi,afr×CC  (III)

where: Xi is the mole fraction of exhaust component i;

Xi,afr is the mole fraction of component i realized in a combustiondevice with low combustion completeness;

Gi,afr is the difference between the mole fraction of component irealized in a combustion device with high combustion completeness andXi,afr; and

CC is the degree of combustion completeness, 0 representing 0%combustion completeness. and 1 representing 100% combustioncompleteness.

As illustrated in FIG. 2, the chemical composition is a function of theair/fuel ratio. However, different compositions can exhibit the sameair/fuel ratio. The fact that the air/fuel ratio does not uniquelydetermine the chemical composition of the emission has previously madeit impossible to use an air/fuel ratio sensor as a chemical compositionmeasuring device.

In a CEGA sensor, the degree of combustion completeness is determined inaddition to the air/fuel ratio. By determining both the air/fuel ratioand degree of combustion completeness, this combination of measurementscan be used to uniquely determine the chemical composition.

With regard to the CEGA sensor illustrated in FIG. 4, oxygen pumpingcurrent (I_(pe)) is measured across electrodes 26 and 82 and can be usedto determine combustion completeness (CC) via the equation:

CC=CC _(max) −G _(cc) ×I _(pe)  (IV)

where: G_(cc) and CC_(MAX) are determined by experimentation.

With regard to the CEGA sensor illustrated in FIG. 5, pumping current(I_(pe)) measured across electrodes 80 and 22 and can be used todetermine combustion completeness (CC) via the equation:

CC=I _(pe) /I _(p)  (V)

CEGA sensors provide several significant advantages over prior artair/fuel ratio sensors such as UEGO sensors. For example, CEGA sensorscan be used to determine the chemical composition of an exhaust from acombustion device as well as the condition of an upstream catalyticconverter, the condition of the catalytic converter being determined byplacing the CEGA sensor downstream of the converter. If the catalyticconverter is working effectively to bring the exhaust into chemicalequilibrium, the potential between the electrodes 26 and 80 in the CEGAsensor will be small. In this regard, the CEGA sensor functions muchlike a second catalytic converter. Accordingly, if the potential betweenthe electrodes 26 and 80 is small, this indicates that the firstcatalytic converter is effectively bringing the emissions into chemicalequilibrium. By contrast, if the potential between the electrodes 26 and80 is large, this indicates that the first catalytic converter isoperating at less than 100% effectiveness. As a result, theeffectiveness of the catalytic converter can be measured as a functionof the I_(pe).

Automotive manufacturers currently use two exhaust sensors to determinethe condition of the catalytic converter, one upstream and onedownstream of the catalytic converter. The stoichiometry of the engineis varied to give a sinusoidal signal to the sensor upstream of thecatalyst. If the sensor downstream of the catalyst sees the samesinusoidal signal then the catalytic converter is not effective. Theadvantage of the described invention is that it removes the necessity ofthe exhaust sensor normally positioned upstream of the catalyticconverter.

The present invention also relates to several methods for operatingceramic sensors as well as devices which can be used in combination withceramic sensors which improve the performance of the sensors.

3. Method for Calibrating Air/Fuel Ratio Sensors

One embodiment of the present invention relates to a method forcalibrating a ceramic sensor which, as one of its functions, determinesan air/fuel ratio. This method can be used in combination with anysensor which calculates an air/fuel ratio including, but not limited toUEGO, NO_(x), and CEGA sensors.

In this embodiment, the accuracy of a ceramic sensor which determines anair/fuel ratio is improved by using a basic relationship between apumping current (I_(p)) and an air/fuel ratio and modifying the use ofthat relationship to calibrate a specific sensor. FIG. 6 illustrates thebasic relationship between the pumping current (I_(p)) in a ceramicsensor and the air/fuel ratio.

According to the method, a ceramic sensor is operated at a constant,known air/fuel ratio. While being operated at a constant, known air/fuelratio, the pumping current (I_(pm)) of the sensor is measured. A basicrelationship which correlates the air/fuel ratio to the pumping current,such as FIG. 6, is then used to calibrate the sensor by comparing themeasured pumping current (I_(pm)) to the expected pumping current forthat air/fuel ratio (I_(p)). A transformation between the measuredpumping current (I_(pm)) and the current that the basic relationshipgives for a known air/fuel ratio is created. During subsequent sensorusage, this transformation is used to modify the measured pumpingcurrent to create a value which is used with the basic relationship toobtain an air/fuel ratio that is accurate for the specific sensor.

A software algorithm can be used to compare I_(pm) versus I_(p) for oneor more air/fuel ratios and produce a look-up table for I_(pm) versusair/fuel ratio which can be used during the operation of the sensor.

This method has the advantage of being computationally simple, thusenabling the calibration to be performed quickly. Because the algorithmfor performing this method is simple, a small amount of memory is neededto store the algorithm. In addition, the algorithm does not requiredetailed knowledge of the characteristics of the sensor.

4. Memory Device For Ceramic Sensors

This embodiment of the invention relates to a semiconductor memorydevice which can be used in combination with or incorporated into aceramic sensor. The memory device can be used in a system which includesthe sensor to control the sensor, calibrate the sensor, and/or monitorthe sensors usage and performance.

FIG. 7 illustrates an embodiment of a memory device coupled to a sensorin a system according to the present invention. As illustrated, thememory device 90 is designed to be attached to the gaseous componentanalyzer 94. The sensor is also attached to the analyzer.

FIG. 8 illustrates an alternate embodiment where the memory device 90 isbuilt into the sensor 92. In this embodiment, the memory device 90 andsensor 92 have a single connector 98 which attaches to the analyzer 94via connector 99.

The memory device can include logic for performing a variety offunctions. For example, the memory device can include logic forcalibrating the sensor. In addition, the memory device can include alook-up table for use with the logic to calibrate the sensor. By using amemory device in combination with a sensor, automated calibration of gassensors can be performed.

For standardized quality control methodologies, it may be required thatthe user calibrate the sensor. Calibration is generally performed at acentral location, after which the sensors are distributed to sites wherethey are used. The memory device of the present invention can includelogic to store field calibration information which can be transferredfrom site to site as the sensor is used. By using the memory device inthis manner, opportunity for calibration information loss or mistakes inits use are significantly reduced.

The memory device can also include logic and memory for storing usageinformation regarding the sensor. For example, the usage memory can beused to record the number of hours that the sensor has been used. Thiswould allow a sensor manufacturer to prorate warranty settlements basedon actual recorded sensor usage. The usage memory can also be used torecord the conditions under which the sensor has been used. Thisinformation would allow a sensor manufacturer to see the conditionsunder which the sensor was used and to use this information for sensordevelopment and/or marketing and for warranty issues should the useroperate the sensor outside of its recommended limits.

The memory device can also include logic which monitors and controls theoperation of the sensor. For example, some sensors can be damaged ifthey are heated too rapidly. The memory device can function to controlhow fast the sensor is heated.

The memory device can also include logic for detecting when the sensoris being used or has been used beyond its recommended limits, e.g.,temperature, time, voltage, etc. The memory device can also include amechanism for warning the user of the improper use or overuse.

5. Mechanism and Method For Compensating For Thermal Load Transients

During thermal load transients, ceramic sensors can experience periodswhere the sensor is not at the desired temperature. These temperatureerrors can reduce the accuracy of the sensor. Even with the use of aclosed-loop temperature control system, significant errors intemperature can often occur. In order to accommodate for these errors intemperature, logic is provided for use with ceramic sensors whichcompensates for errors that can occur due to temperature transients thatare not corrected by a closed-loop control system. By correcting forthese errors in temperature, the accuracy of measurements from ceramicsensors is improved.

FIG. 9 is a graph plotting current (I_(p)) from a ceramic sensor (yaxis) as a function of sensor impedence. The sensor is being exposed toa constant quantity of combustion emissions components and, as such, thecurrent (I_(p)) should be constant. The temperature of a sensor can bedetermined based on the impedence of the sensor (x axis). Impedence ispreferably measured near the time that the sensor's output signal(s) ismeasured.

According to one embodiment, the temperature of the sensor is measuredbased on the sensor's impedence. The sensor's output is then correctedbased on the detected temperature if the sensor's temperature is foundto deviate from the desired temperature, according to the followingequation:

I _(corrected) =I _(measured) −I _(correction) =I _(measured) −G×BR×(R_(actual) −R _(target))  (VI)

where:

I_(correction) is the correction to the pumping or dissociation currentsmeasured in a ceramic exhaust gas sensor;

G is the gain of the sensor at the particular operating point;

BR is the slope of the I_(p) versus sensor impedence curve in thevicinity of the target impedence (R_(target)) (BR is normalized by theaverage sensor gain);

R_(actual) is the actual sensor temperature (given here in terms of animpedance); and

R_(target) is the target sensor temperature (given here in terms of animpedence).

In an alternative embodiment, the gain of the sensor is modified by theactual sensor temperature according to the following equation:

G _(corrected) =G×(1+G×MR×(R _(actual) −R _(target))  (VI)

where:

MR is the sensitivity of the gain to a sensor temperature error (MR isnormalized by the average sensor gain).

The present invention also relates to a sensor which can include logicfor receiving the resistance data from the sensor and adjusting thesensors output based on the above method.

6. Mechanism and Method For Timing Sampling To Increase Signal-To-NoiseRatio

A need exists to maximize the signal-to-noise ratio of sensors. FIG. 10Aillustrates a noiseless signal (I_(p2)) in the sense that extraneousnoise has been eliminated. Applicants have determined the existence ofseveral sources of noise which mask this signal and have designed atiming pattern for taking sensor measurements which significantlyreduces the effects of these sources of noise. By incorporating thistiming pattern for taking measurements (i.e., sampling), thesignal-to-noise ratio of a ceramic sensor was improved by a factor of20.

Applicants have detected the presence of leakage current from the heaterof a ceramic sensor to its sensing element. FIG. 10B illustrates thecycling of voltage to the heater. As illustrated, the heater transitions101 from an off state 105 to an on state 103 and transitions 107 fromthe on state 103 to the off state 105. FIG. 10C illustrates how thesignal 111 illustrated in FIG. 10A is modified as a result of theleakage current. As can be seen in FIG. 10C, the leakage current causesthe signal amplitude to increase 115 relative to signal 111 when theheater is on 103 and also introduces a noise effect 113 where the signalamplitude changes over time during an on or off state. This leakagecurrent has been found to vary from sensor to sensor and change withsensor temperature.

To minimize the effect of heater leakage noise, measurements arepreferably taken from the sensor when the heater is off or after theeffects of the leakage current have reached steady state, mostpreferably just prior to turning the heater on. These time periods areillustrated in FIG. 10C as 119 and 121 respectively.

Applicants have also detected noise generated due to coupling betweenthe heater wires and sensing element's wires. To minimize the effect ofnoise generated by coupling between the heater wires and the sensingelement's wires, measurements are preferably taken just beforetransitions in the heater's voltage occurs. These transitions areillustrated in FIG. 10B as 101 and 107 and their noise effects areillustrated in FIG. 10D as 117.

Another source of noise that has been detected is leakage current due tothe use of an impedence method for measuring a sensor's temperature.FIG. 10E illustrates the timing of the impedence method where the noiseeffect is illustrated as 109. To minimize the effect of leakage currentdue to the impedence method, measurements are taken just before theimpedence method is to occur.

A method is provided according to the present invention for takingsensor measurements based on a timing pattern which is designed to avoidthe effects of these different sources of noise. This method can involveconsideration of the heater's duty cycle on the timing pattern. Forexample, sensor measurements can be timed to not coincide with thedelivery of current to the heater.

As illustrated in FIGS. 10F and 10G, at low heater duty cycles, noise117 dominates the period when the heater is on 103. It is thereforepreferred that sampling be done when the heater is off when the heaterduty cycle is low.

As illustrated in FIGS. 10H and 101, at high heater duty cycles, noise117 dominates the period 105 when the heater is off. It is thereforepreferred that sampling be done when the heater is on when the heaterduty cycle is high. In a preferred embodiment, the system in which thesensor is used includes logic for determining whether the heater dutycycle is low or high and for selecting the sampling times based on theduty cycle.

7. Method And System Involving Use Of Regulated Voltage-type Heater WithCeramic Sensor

Pulse-width modulated (PWM) heater controllers have traditionally beenused with ceramic sensors. These heater controls serve to cycle theheater between on and off modes and thus include on-to-off and off-to-ontransitions. This type of controller presents the operationaldisadvantages of having a long duty cycle at low voltages and exhibitingcoupling between the heater wires and the sensing element wires duringthe on-to-off and off-to-on transitions of the heater.

According to this embodiment of the invention, a regulated voltage-typeheater may be used with a ceramic sensor. Regulated voltage-type heatershave not used with ceramic sensors due to the greater complexity oftheir design and the observance of current leakage from continuousheater voltage which affects the pumping and dissociation currentreadings. In this embodiment, the contribution to the pumping and/ordissociation current by the heater is measured at regular intervals andsubtracted out in order to compensate for this source of noise. Thisenables more highly accurate measurements by allowing for more time forsample averaging as compared to PWM heater controllers. In addition,faster sensor start-up is enabled because the controller does not haveto wait until the heater effects have decayed to steady-state values.

FIGS. 11 and 12 illustrate a NO_(x) sensor with a regulated heatervoltage 125. The temperature of the sensor can be measured by one of avariety of techniques including, for example, sensor impedance method,heater resistance, and thermocouple. Measurement of the temperature ofthe sensor is then used to modify the regulated voltage 125. The circuitalternatives between the two configurations, defined by the position ofswitch 111.

Configuration 1 is shown in FIG. 11. As illustrated in this figure, thedissociation voltage V_(s2) 113 is applied across electrodes 117 and 119causing the dissociation of oxygen-containing species in thedissociation cavity. In this configuration, the measured current I_(p2)is due to the dissociation and leakage 121 of current from the heater123.

Configuration 2 is shown in FIG. 12. As illustrated, the dissociationvoltage V_(s2) 113 is not applied across electrodes 117 and 119. In thisconfiguration, the measured current I_(p2)′ is due only to leakage 121from the heater 123.

The difference between the measured currents I_(p2) and I_(p2)′(I_(p2)−I_(p2)) is independent of the effect of the heater and is usedto determine the concentrations of components in the exhaust 23.

8. Method And Logic For Calibrating Sensors

For a two-cavity sensor, the basic relationship between the measuredpumping current I_(p2) 57 and the amount(s) of oxygen-containing speciesin the exhaust 23 is:

I _(p2) =I _(p2) =C+KX _(i)

where:

I_(p2)=current due to pumping out O₂ from the second cavity (where thedissociation occurs) 48 that enters the second cavity from the firstcavity 28 (the cavity prior to the cavity where the dissociation occurs)via the second diffusion layer 51+current due to pumping out oxygen fromthe second cavity 48 that is created in second cavity from dissociatingoxygen-containing species (ex. NO_(x), CO₂, H₂O). [VIII]

C is a constant;

K is a constant set by the propensity of the sensor to dissociatespecies i;

X_(i) is the concentration of oxygen containing species in the exhaust23;

It is standard practice to control the O₂ content of the first cavity toa constant amount and thus fix the first term in Equation VIII to aconstant quantity C.

It is also standard practice to have conditions (ex. temperature, oxygenconcentration, electrode composition) in the first cavity not conduciveto the increase or reduction of the amount(s) of oxygen-containingspecies to be measured.

It is also standard practice to have conditions in the second cavityconditions (ex. temperature, oxygen concentration, electrodecomposition) conductive to the dissociation of the oxygen-containingspecies to be measured.

It is standard practice to choose the level of the dissociation voltageV_(s2) 55 of the second cavity to select those oxygen-containing speciesto dissociate. At a low pumping voltage, primarily one species willdissociate. As the pumping voltage is increased, additional species willdissociate contributing to a greater I_(p2). If a series of pumpingvoltages are used, the relative contribution of the differentdissociated species, and hence the relative amounts of the dissociatingspecies in the exhaust can be resolved.

The reasons for these standard practices is to make the first term inEquation VII a constant and the second term a function of theoxygen-containing dissociating species, ie.:

I _(p2) =C+K×Xi  [IX]

or

Xi=(1/K)×(I _(p2) −C)

or

Xi=H×(I _(p2) −C)

or

Xi=H×I _(p2) −A

where:

I_(p2)=pumping current in the cavity where the dissociation occurs; and,

C=a constant; and,

K=is a constant set by the propensity of the sensor to dissociatespecies i; and,

Xi=the concentration of dissociating oxygen-containing species i in theexhaust; and

H=1/K; and,

A=C/K.

Equation IX represents a prior art method for measuring the amount(s) ofoxygen-containing species in an exhaust. It assumes that just oneoxygen-containing species is dissociating. If multiple oxygen-containingspecies are dissociating then Xi in Equation IX will have to be replacedby ΣXi and different dissociation voltages V_(s2) will have to be usedresolve the contributions of the different dissociating species toI_(p2).

One shortcoming of Equation IX is that it is difficult if not impossibleto control without error the amount of oxygen entering the secondcavity. Therefore, due to imperfections in sensor design and control, Cwill vary with exhaust composition.

A second shortcoming of Equation IX is that it is difficult if notimpossible to avoid reactions that increase or decrease the amounts(s)of oxygen-containing species prior to their arrival in the second cavity(where they are measured). These reactions result in errors inmeasurements of the species. The magnitude of these errors can vary withexhaust composition because the rates of these reactions can vary withexhaust composition.

A third shortcoming of Equation IX is that it is difficult if notimpossible to avoid the reactions of some oxygen freed by dissociationin the second cavity with molecules such as CO and HCs. These reactionsresult in errors in the measurements of the species. The magnitude ofthe errors can vary with exhaust composition because the amounts of theinterfering species can vary with exhaust composition.

The present invention relates to a method for mitigating the effects ofthe above shortcomings on sensor accuracy by using a knowledge of thecomposition of the exhaust. The present invention also relates tosensors and sensor systems which incorporate logic for performing themethod.

As can be seen from the graph illustrated in FIG. 2, the composition(ie. amounts of CO, O₂, HC) of the exhaust of combustion devices isstrongly influenced by the air-fuel ratio of the combustion device (ex.engine) producing the exhaust. The air-fuel ratio of the combustiondevice is a function of the pumping current I_(p1) 53 of the firstcavity via prior art.

Using these relationships, the following equation has been derived:

Xi=H(I_(p1))×(I _(p2) −C(I_(p1)))  [X]

where:

H and/or C are not constants but rather functional relationships of thepumping current I_(p1) of the first cavity. These relationships can bedetermined experimentally.

The pumping current I_(p1) used in Equation X is preferably normalizedto its value in a common and stable gaseous environment (ex. I_(p1) inair). Accordingly, Equation X can be recast as:

Xi=H(I_(p1) /I _(p1n))×(I_(p2) −C(I_(p1) /I _(p1n)))  [XI]

The advantage of the construct of Equation XI is that the functionalrelationships between H or C and I_(p1)/I_(p1n) do not degrade withsensor degradation. This is because the relationship between I_(p1) andair-fuel ratio scales with the relationship between I_(p1n) and thecommon gaseous environment.

Using Equations X and XI, methods have been developed for determiningthe amounts of each oxygen-containing species (ex. NO_(x), CO₂, H₂O) ina sample of combustion exhaust using any sensor that dissociates eachspecies and uses the quantity of oxygen molecules produced by thedissociation as an indication of the amount of each species in theexhaust. One type of sensor which may be used in this method aretwo-cavity sensors, such as the one illustrated in FIG. 3. However, themethod is not intended to be limited to two-cavity sensors. Rather, themethod can be applied to a single-cavity, or three-cavity, or any otherconstruct of sensor that uses dissociation to determine the amounts ofoxygen-containing species.

According to one embodiment, the method is performed by applying agaseous emission to the sensor; measuring a pumping current in a firstcavity of the sensor which has a functional relationship to an air/fuelratio of the gaseous emission; measuring a pumping current in a secondcavity of the sensor which has a functional relationship to an amount ofoxygen-containing species in the gaseous emission and the air/fuel ratioof the gaseous emission; and using a combination of the measured pumpingcurrents of the first and second cavities to measure an amount ofoxygen-containing species in the gaseous emission.

One advantage of the present method is a decrease in errors caused byvariations in exhaust composition and sensor degradation, thus improvingthe sensor's accuracy. This method allows accurate calibration of thesensor using a simple combination of model gases. The use of model gasessimplifies sensor calibration which makes the calibration less costlyand quicker to perform, while retaining the accuracy of a calibrationusing actual combustion exhaust. This method permits sensor fieldcalibration in a variety of environments which broadens the range ofapplications for the sensor.

Sensors are not conventionally calibrated in actual combustion exhaust.Instead, “model gases” of simplified compositions are generally used. Amodel gas is made by blending gases from tanks of specific molecules.For example, a simple model gas composition of NO_(x), O₂, CO, and N₂might be used to calibrate a sensor to measure NO_(x). The problem withsuch calibration procedures is that it ignores the possible effects onthe accuracy of the sensor of species that are in the exhaust but arenot in the model gases. Such absent species, when in the exhaust, mayeither consume dissociated oxygen or cause more oxygen to bedissociated. The result being that a model gas calibration may notresult in a calibration that is accurate in actual exhaust.

Since Equations IX, X, and XI are not limited to either model gases oractual exhaust, a relationship exists between the functions H and Cdetermined in a model gas calibration and those determined in an exhaustgas calibration. For example, in one embodiment functions H and Cdetermined in a model gas calibration are used to accurately calculatethe amount of species i in an actual exhaust according to the equation:

Xi=G×H×(I_(p2) —M×C)  [XII]

where:

G and M are corrections to H and C when going from measuring species iin a model gas to measuring species i in an exhaust; and

H and C are determined using model gases.

G, H, M, and C are preferably expressed as functions of I_(p1)/I_(p1n).G and M are preferably determined once for a given construction ofsensor and type of fuel combusted. If species i is to be measured in amodel gas, G and M are set to 1. The advantage of the method of EquationXII is that it allows simple model gas calibrations to give exhaust gascalibration accuracy.

In a further embodiment of the present invention, the relationshipexpressed. in Equation XII is used to enable field calibration ofsensors. In general, it is desirable that instrumentation be able to besimply recalibrated while in use. Such calibrations are called “fieldcalibrations”. Field calibrations are typically two-point calibrations:one point at zero amount of the species to be measured and one point ata value greater than the maximum amount of the species to be measured.In order to perform field calibrations, Equation XII is modified togive:

Xi=G×H×SPAN×(I_(p2) −M×(C−ZERO))  [XIII]

where:

SPAN=Xi/(G×H×(I_(p2) −M×(C−ZERO)))

at the upper calibration point; and ZERO=C−I_(p2) /M when Xi=0.

According to Equation XIII, the result of field calibration is thedetermination of the quantities SPAN and ZERO to be used in EquationXIII. The advantage of this method is that field calibration can beperformed in either model gases or actual exhaust. If field calibrationis performed in model gases, G and M are set to 1 in the equations forSPAN and ZERO. If field calibration is performed in exhaust, G and Massume their values as determined.

According to one embodiment, the method of field calibration isperformed by applying a gaseous emission having a known amount ofoxygen-containing species to the sensor; measuring a pumping current ina first cavity of the sensor which has a functional relationship to anair/fuel ratio of a model gas; measuring a pumping current in a secondcavity of the sensor which has a functional relationship to the amountof oxygen-containing species in the gaseous emission and the air/fuelratio of the gaseous emission; and using a combination of the measuredpumping currents of the first and second cavities and the known amountof oxygen-containing species in the gaseous emission to calibrate thesensor.

9. Method And Logic For Compensating For Rapid Emission CompositionTransients

This embodiment provides a method for minimizing the effect of rapidemission composition transients on the accuracy of multiple cavityexhaust sensors while minimizing the associated additional costs.

FIG. 13 illustrates signals from a two cavity exhaust sensor duringengine emission composition transients. The sharp transients in measuredNO_(x) are not real but rather are caused by the delay in the firstcavity's ability to control the level of oxygen in that cavity. Shouldthe level of oxygen become greater or less than its target value, theperceived concentration of NO_(x) as measured in the second cavity,becomes greater or less than it actually is.

The method of this embodiment involves controlling when NO_(x) is and isnot measured such that NO_(x) is measured when the measurement isaccurate and is not measured when the measurement is inaccurate.

As shown in FIG. 14, Vs has a very abrupt change in value with air/fuelratio and oxygen concentration in the vicinity of the stoichiometricpoint. Multiple cavity sensors such as two-cavity sensors are typicallyoperated on or near the stoichiometric point and are used to control theconcentration of oxygen in the first cavity to a target value. Duringrapid emission composition transients, the concentration of oxygen inthe first cavity will go off-target and this will be reflected in anoff-target V_(s) value. This embodiment uses V_(s) information tocontrol the compensation logic.

FIG. 15 illustrates a software flowchart for the technique. The softwareflowchart contains four conditional loops: threshold loop, peak detectloop, timer loop, and recovery loop.

The threshold loop determines whether or not to compensate the measuredNO_(x) value based on a difference between the value of Vs and thetarget value, shown in the flowchart to be 0.45 V. When Vs diverges fromthe target value by more than a predetermined value, shown in theflowchart as β, then the peak detect loop is entered. In the peak detectloop, the previously determined value of NO_(x) is used instead ofsubsequent samples. Within the peak detect loop, a maximum deviation ofVs is monitored for. When a maximum deviation of Vs relative to thetarget value is reached, a timer is started for a predetermined timeperiod, shown as y, and the timer loop is entered.

For the predetermined time period, the previously determined value forNO_(x) is used until either the predetermined time period expires or thedifference between Vs and the target value increases. When the timeperiod expires, the recovery loop is entered. Meanwhile, if thedifference between Vs and the target value is found to be increasing,the threshold loop is entered.

In the recovery loop, the sampled NO_(x) value is used until Vs deviatesfrom the target value less than β or Vs begins to diverge from itstarget value. When either of these events occur, the threshold loop isentered.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than limitingsense, as it is contemplated that modifications will readily occur tothose skilled in the art, which modifications will be within the spiritof the invention and the scope of the appended claims.

What is claimed is:
 1. A method for quantifying a component of a gasemission comprising: taking a sensor including a first pumping cellwhich includes first electronics which maintain a first chemicalenvironment adjacent the first pumping cell and measures a first pumpingcurrent, and a second pumping cell which includes second electronicswhich maintain a second chemical environment adjacent the second pumpingcell and measures a second pumping current; contacting the sensor with agas emission; measuring the first and second pumping currents; andquantifying a component of the gas emission using the equation Xi=H(I_(p1))×(I _(p2) −C(I _(p1)))  where Xi is the concentration of thecomponent being quantified; I_(p1) is the first pumping current; I_(p2)is the second pumping current; H(I_(p1)) is a non-constant function ofthe first pumping current; and C(I_(p1)) is a constant.
 2. A methodaccording to claim 1 wherein the sensor is an oxygen-containing speciesdetection sensor.
 3. A method according to claim 1 wherein the sensor isa NO_(x) sensor.
 4. A method according to claim 1 wherein the componentmeasured is an oxygen-containing species.
 5. A method according to claim1 wherein the component measured is NO_(x).
 6. A method according toclaim 1 wherein the component measured is CO₂.
 7. A method according toclaim 1 wherein the component measured is H₂O.
 8. A method according toclaim 1 wherein the component measured is CO.
 9. A method according toclaim 1 wherein the component measured is O₂.
 10. A method according toclaim 1 wherein an electrode of the first pumping cell has a differentcatalytic activity than an electrode of the second pumping cell.
 11. Amethod according to claim 1 wherein at least one of the first and secondpumping cells pump oxygen.
 12. A system for quantifying a component of agas emission comprising: a sensor comprising: a first pumping cell whichincludes first electronics which maintain a first chemical environmentadjacent the first pumping cell and measures a first pumping current; asecond pumping cell which includes second electronics which maintain asecond chemical environment adjacent the second pumping cell andmeasures a second pumping current; a mechanism which measures the firstand second pumping currents; and a mechanism which quantifies acomponent of the gas emission using the equation Xi=H(I _(p1))×(I _(p2)−C(I _(p1)))  where Xi is the concentration of the component beingquantified; I_(p1) is the first pumping current; I_(p2) is the secondpumping current; H(I_(p1)) is a non-constant function of the firstpumping current; and C(I_(p1)) is a constant.
 13. A system according toclaim wherein the sensor is an oxygen-containing species detectionsensor.
 14. A system according to claim 12 wherein the sensor is aNO_(x) sensor.
 15. A system according to claim 12 wherein the componentmeasured is an oxygen-containing species.
 16. A system according toclaim 12 wherein H(I_(p1)) and C(I_(p1)) are for measuring NO_(x).
 17. Asystem according to claim 12 wherein H(I_(p1)) and C(I_(p1)) are formeasuring CO₂.
 18. A system according to claim 12 wherein H(I_(p1)) andC(I_(p1)) are for measuring H₂O.
 19. A system according to claim 12wherein H(I_(p1)) and C(I_(p1)) are for measuring CO.
 20. A systemaccording to claim 12 wherein H(I_(p1)) and C(I_(p1)) are for measuringO₂.
 21. A system according to claim 12 wherein at least one of the firstand second pumping cells pump oxygen.